One common method for preparing powdered inorganic pigments comprising one or more metal compound is to intimately admix the powdered components and thereafter calcine the powdered mixture at high temperatures for extended periods such as, for example, for up to 20 or 30 hours or more. In some instances, modifiers and diluents can be incorporated into the mixture to modify and generally improve the properties of the pigments. After the pigments are formed by calcining, the pigments generally are crushed and ground to a fine powder by techniques such as ball milling, wet or dry attrition milling, etc.
The preparation of inorganic pigments comprising two or more metals from solid mixtures is well known and described in the prior art such as in U.S. Pat. Nos. 3,748,165; 2,644,767; 2,269,508; 2,416,064; 2,309,173; 2,068,294 and 4,230,500. The '165 patent describes such procedures for preparing nickel-cobalt aluminate pigments, and the '508 patent describes the preparation of zinc aluminate pigments wherein a mixture of zinc oxide and aluminum hydrate is heated to a temperature of 750.degree.-950.degree. C. until the desired zinc aluminate is formed. U.S. Pat. No. 2,644,767 describes the preparation of pigments of the cobalt alumina type.
U.S. Pat. No. 2,416,064 describes chromite ore pigments obtained by heating a mixture of powdered chromite ore and an alkaline earth material such as, slaked lime in an oxidizing atmosphere, and after drying, the mixture is calcined. Black ceramic pigments are described in U.S. Pat. No. 2,309,173. The pigments which are of the copper-chrome type are obtained by heating solid mixtures of the oxides in the presence of certain addition agents or mineralizers including, for example, zinc oxide, cadmium oxide, small amounts of phosphorus acid or compounds yielding P.sub.2 O.sub.5 on calcination, molybdenum oxide, tungsten oxide, vanadium oxide, etc.
U.S. Patent 2,068,294 describes pigments comprising mixed crystals containing in their lattice construction zinc oxide, at least one oxide of a metal of the fourth group of the periodic system consisting of Si, Zr, Sn and Ti, and at least one coloring oxide of a metal of the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu and U. Pigments are prepared by combining at a temperature between about 800.degree. C. and 1100.degree. C., zinc oxide, at least one oxide of the metals from the fourth group identified above and at least one coloring oxide of a metal of the group identified above.
U.S. Patent 4,230,500 describes pigments consisting essentially of bismuth vanadate of monoclinic structure, bismuth phosphate of monoclinic structure and aluminum phosphate of orthorhombic structure. In the case of yellow and orange-yellow pigments, the pigments also comprise a compound based on Bi.sub.2 O.sub.3 and V.sub.2 O.sub.5 with a characteristic X-ray diffraction pattern. The pigments are prepared by calcining, in the presence of air, BiPO.sub.4, Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 or compounds which generate Al.sub.2 O.sub.3 and V.sub.2 O.sub.5 during the calcination.
The incorporation of about 1-5% of metallic cyanides in black copper-chrome pigments is described in U.S. Pat. No. 2,264,749 as improving the utility of the pigment as a graining pigment for decorative printing on vitreous enamels. The pigments are prepared by thoroughly milling the cyanide compound with the copper-chrome pigment.
Inorganic pigments having a spinel structure and having the formula AB.sub.2 O.sub.4 wherein A is at least one of Co, Ni, Cu, Zn, Cd, Mg, Mn and Fe, and B is at least one of Al, Cr, Fe, Ga, In, La or V is described in U.S. Pat. No. 4,696,700. These pigments reported to be in the form of platelets are obtained by mixing the components with each other in known manner and calcining at a temperature above 1000.degree. C. while continuously moving the mixture during calcination and with further intermixing such as in a rotary kiln or high temperature cyclone. Preferably the mixture of components is prepared by mixing the solid component under wet conditions in order to obtain a homogeneous mixture which is then dried in an oven, ground to reduce the particle size and then calcined.
U.S. Pat. No. 3,935,128 describes improved copper chromite catalysts having high density, low monovalent ion content, high activity and resistance to poisoning. These copper chromite catalysts are prepared by reacting a soluble copper salt with chromic acid or a dichromate in solution in the presence of ammonia and urea to precipitate a copper-chromium-nitrogen complex which is washed, dried and calcined. The patent discloses that the employment of urea as a processing aid results in the formation of a catalyst having a unique structure and the above-identified desirable properties. The catalyst also has large particles which can be washed free of reaction products to improve yields. The calcination of the precipitate may be conducted at temperatures of from about 200.degree. C. to about 1000.degree. C. and more generally at temperatures of from 300.degree. C. to about 475.degree. C. Comparative examples are included in the '128 patent which do not contain any urea, and the properties of the resulting catalysts are compared.
U.S. Pat. No. 2,811,463 describes inorganic black pigments comprising manganese, copper and iron but which are substantially free of chromium. The pigments can be made by directly calcining the oxides or compounds of manganese, copper and iron which yield their oxides on calcination, or the pigments may be prepared by coprecipitating the pigment and then calcining the precipitate. In either case the calcined pigment comprises 20 to 80 parts by weight of MnO, 20 to 80 parts by weight of CuO and 5 to 50 parts of FeO. Calcination temperatures of 800.degree. F. to 1400.degree. F. are utilized. This patent teaches that chromium is to be avoided because it decreases the tinting strength of the pigment.
U.S. Pat. No. 2,248,021 also relates to black pigments which are high chrome copper-chrome pigments which may also contain up to about 5% by weight of MnO.sub.2. The MnO.sub.2 is reported to make the black less bluish and more brownish in color. The mole ratio of the chromium to copper in the pigments is from 1.5:1 to 2.5:1.
U.S. Pat. No. 4,643,772 describes the process of the preparation of a mixed phase pigment based on iron oxide and chromium oxide by heating a mixture of the oxides, hydroxides or oxide hydroxides of iron and chromium at from 600.degree. C. to 1100.degree. C., comminuting the product and working it up in a conventional manner. In particular, the process involves dissolving a chromium salt in a neutral suspension of transparent iron oxide in water and thereafter adding an alkali to the mixture whereby chromium hydroxide is precipitated onto the transparent iron oxide which is then calcined and wet-milled in a ball mill. In Example 1, the ball-milled product is subsequently milled in a turbomill.
Although pigments prepared by prior art techniques provide desirable and useful properties, the search for pigments having greater or higher tinting strength continues. Also, while various pigments have been used widely to color various substrates for many years, the use of pigments in formulations intended for long term outdoor exposure has increased the demands placed on the pigments. For example, the use of pigments in prefinished building siding requires that the pigment exhibit long-term resistance to fading by virtue of exposure to sunlight under the usual outdoor exposure. Thus, pigments exhibiting better weatherability and higher tinting strength are of significant interest to the industry.